1. Technical Field
This invention relates to a quantum-well type semiconductor laser device capable of oscillating at a wavelength between 1.3 and 1.6 .mu.m.
2. Background Art
A semiconductor laser device with quantum-well type structural features has a low threshold current density, when compared with a semiconductor laser device having active layers of bulk crystals, and is capable of reducing its internal loss to show a narrowed far-field angle (beam spreading) by reducing the number of quantum-well layers. These and other characteristics of a quantum-well type semiconductor laser device make it a promising candidate for popular use in the future.
There have been known semiconductor laser devices of distribution feedback type having a quantum-well structure that operate in a single longitudinal mode. FIG. 7 schematically illustrates such a semiconductor laser device having band gaps.
A semiconductor laser device as shown in FIG. 7 comprises a number of layers formed on an n-InP substrate 1 and arranged in a given manner, including an n-InP clad layer 2, a non-doped GaInAsP optical confinement layer 3, a multiple quantum-well layer 4 which comprises a plurality of alternately arranged non-doped GaInAsP quantum-well layers 5 and GaInAsP barrier layers 6, a non-doped GaInAsP optical confinement layer 7 in which a diffraction grating 8 is formed, a p-InP clad layer 9, a p-GaInAsP contact layer 10 and a buried layer 11 which is constituted by a p-InP layer 11a and an n-InP layer 11b along with an n-side electrode 12 arranged under the substrate 1 and a p-side electrode 13 arranged on the buried layer 11. The layers of FIG. 7 have the following specified characteristic values.
______________________________________ optical confinement layer 3 band gap wavelength - 1.3 .mu.m, thickness - 1,200.ANG. quantum-well layer 5 band gap wavelength - 1.67 .mu.m, thickness - 100.ANG. barrier layer 6 band gap wavelength - 1.3 .mu.m, thickness - 150.ANG. optical confinement layer 7 band gap wavelength - 1.3 .mu.m, thickness 1,200.ANG. primary diffraction grating 8 pitch - 2,400.ANG. height - 500.ANG. ______________________________________
The paper listed below, or Paper 1, reports that the spectral purity of a semiconductor laser device having a quantum-well structure can be improved by reducing the number of quantum-well layers which are active layers.
______________________________________ Paper 1: IEEE, JOURNAL QUANTUM ELECTRONICS, Vol. QE-21, No. 10, pp. 1666-1674, 1985. ______________________________________
If, however, the number of quantum-wells of a semiconductor laser device having a structure as described above is reduced to 1 or 2, the optical confinement factor of the device is reduced to significantly decrease the gain of the device and consequently make the threshold carrier density required for oscillation rather high.
If, by turn, the threshold carrier density for oscillation of such a device is high in a longwave region, the efficiency of light emission of the device is inevitably deteriorated by a non-light emitting mechanism inherent in the device, typically involving Auger effect or inter-valence band absorption. Consequently, the oscillation threshold current can raise remarkably to allow oscillation to take place only at the second quantum level or in the optical confinement layers owing to high carrier injection.
Thus, the spectral purity of any known semiconductor laser device cannot be improved by simply reducing the number of quantum-well layers.
The paper below, or Paper 2, discloses a semiconductor laser device that comprises four quantum-well layers with a band gap wavelength of 1.67 .mu.m and a thickness between 50 and 250.ANG. and GRIN-SCH layers with respective band gap wavelengths of 14 .mu.m, 1.33 .mu.m, 1.25 .mu.m and 1.14 .mu.m and a thickness between 150 and 300.ANG., where the difference between the band gap of the quantum-well layers and that of the innermost GRIN-SCH layer is 143 meV.
______________________________________ Paper 2: Appl. Phys. Lett., Vol. 55, No. 22. pp. 2283-2285, 1989. ______________________________________
In order for a semiconductor laser device of the above described type to oscillate at the first quantum level, the above band gap difference needs to be greater than 160 meV. Therefore, the device having a band gap difference of 143 meV is accompanied by the problem of oscillation only at the second quantum level and a high threshold current density.
Moreover, Paper 2 does not disclose any technological improvement for reducing the internal loss, narrowing the far-field angle and improving the temperature dependency of the oscillation wavelength of the semiconductor laser device.